A piezoelectric stack is a component which executes a mechanical movement as a result of an electrical voltage being applied. A piezoelectric stack comprises a plurality of material layers, which react to the application of an electric field, and a plurality of electrode layers, wherein each material layer is arranged between two of the electrode layers. The material layers are usually formed from ceramic.
The material of the material layers is ferroelectric and, during its production, is polarized in one electric field by being heated above the ferroelectric Curie temperature and then cooled down. This process during production is called a polarization process. The dipole moments (Weiss domains) which are initially oriented randomly are oriented substantially in parallel as a result.
Operating the piezo stack during regular operation at a field strength which is smaller than the field strength in a polarization process results, over the course of time, in a depolarization (return orientation) of the dipole moments, which are also called domains. The same effect occurs when the piezo stack is subjected to alternating temperatures, even if no electric field is applied. This effect leads to a change in length of the piezo stack.
In this connection, regular operation is understood to mean operation of the piezo stack under the conditions of its intended use. The intended use of the piezo stack may be, for example, the use within an actuator of a (fuel) injection valve—or: (fuel) injector, for example within a motor vehicle. In this case, regular operation is understood to mean normal (regular) operation of a vehicle
When the piezo stack is used as an actuating element, for example as an operating element in injection valves of an extremely wide variety of types of engine for motor vehicles, this leads to a change in the zero point position. Therefore, an idle stroke, which can have an adverse effect on the functioning and/or efficiency of an engine, can occur when a piezo stack is used in an injection valve.
Particularly when said piezo stack is used as an actuating element for an injection valve (injector), the described effects lead to the dynamic variation in the change in length of the piezo stack having to be taken into account in order to ensure functional reliability of the injector. In order to be able to ensure a zero point position which results from a defined setpoint length of the piezo stack, hydraulic movement compensators or circuits, for example, are used, these allowing operation of the piezo stack with a reversed polarity. A further known variant detects the deviation between a change in length and corrects a length of the piezo stack which differs from the setpoint length by compensation of the idle time of the movement being compensated for by means of changed drive parameters.